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PoEM: Predicting RL Outcomes from Existing Policies
Kimia Hamidieh, Giannis Daras, Antonio Torralba
arXiv:2609.30226v1A major bottleneck in reinforcement learning is that every new reward usually means running a fresh, expensive training loop from scratch. This paper asks a simpler question: if we already have a set of models trained on different rewards, can we predict what RL would produce for a new reward without retraining? The authors introduce PoEM, which treats existing post-trained policies as a basis. In the ideal case, if the new reward is just a combination of old ones, the new policy can be approximated as a matching combination in log space. More surprisingly, they find that even when rewards are not explicitly related, the learned policies often still lie near a low-rank subspace. That means the new policy can be estimated from existing ones by fitting a small set of coefficients from reward or policy outputs. If this works broadly, it could make policy iteration far cheaper, faster, and more modular for language and vision models that need continual post-training.
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